Some tribes are Shamanistic, while some tribes have the Voodoo Religion.

The Darkspear weren't Shamanistic, they learned Shamanism from the Orcs and Taurens.

Shamanic totems were crafted by the trolls and were later adapted by the other Shamans.

I've been wondering about this too, since I have a troll shaman who still practices voodoo, and I'm not sure how much of that she'd have to hide (or whether it's relatively accepted). WoWWiki has some conflicting information, although I've never heard it said outright that they gave up voodoo entirely, just the parts of it that would disturb their allies (going from cannibalism to disrupt the souls of their enemies to equally bizarre but less "ewwww"-factor methods).

Vol'jin, at the very least, would have to be an open practitioner. He's a shadow hunter, and if I'm remembering correctly, they're almost to the loa as paladins are to the Light. Going through quests, there's actually a few trolls who openly practice non-malicious voodoo.

As was mentioned above, since shaman have a few spells that originally belonged to shadow hunters (including the upcoming Hex, I believe), I've sort of come to the conclusion that as the trolls were taught the orcish version of shamanism, they probably influenced the orcs with their traditions, and the tauren mixed in theirs and picked up everyone else's, and we have the class we know today. In which case a troll witch doctor would probably still fall under the designation of "shaman" to most of the Horde--if it quacks like a duck, ect. In the same way Draenei shaman haven't really separated from the Light as far as I've heard, I imagine troll shaman would be as likely to still worship the loa. But as I said, I've found a lot of contradictory information.

@David Bowers

The Darkspear didn't completely abandoned Voodoo and Loa worship, they just abandoned the darker practices of their religion like cannibalism and ritual sacrifices.

That's why the still have the Priest class. They still have Voodoo as their religion.